This invention relates generally to improvements in electrical switches and, more particularly, to new and improved compilations of switching elements wherein assemblies of switches are used to provide an enhanced interface between a user and other components of equipment or machinery.
The term "membrane switch" is commonly used to refer to a multilayered device having a series of discrete mechanical switching elements that can be operated independently of each other by applying to the outermost layer at a given switch site a force which is sufficient to make or break the electrical connection of a particular switch element. Most often, membrane switches are designed for intended use as keyboard, key pad, or front panel interfaces to provide instructions from a user to operate various items, such as computers, manufacturing equipment and vending machines. Accordingly, the force required to activate or deactivate any particular switch element is supplied by a finger of a user.
Individual switches can be provided so that the activating force brings the electrical contacts which complete the electrical circuit into communication and thus actuates or closes the switch, providing an electrical signal for use by other circuitry. Alternatively, the switch units can be configured so the electrical circuit normally is completed, and the communication of the contacts is not disrupted until a force is applied to separate the contacts. Combinations of normally open and normally closed switches can be provided in a single membrane switch to best accommodate the requirements of the circuitry supplied by the signals from the switch elements. Membrane switches also are known that include an amalgam of switch types, each of which may comprise two or more switches that are ganged together at a switch site, so that multiple switch signals are sent to the target circuitry when a force is mechanically applied to only one key.
In order for a membrane switch to serve most effectively as an interface device with a human operator, it is desirable that the user be able to sense with a finger when sufficient activating force has been applied to close or open an individual switch. "Tactile feel," "tactile response," or "tactility" are phrases that typically are used to describe this feature of a membrane switch. Generally, such tactility can be incorporated into a switch via two principle design attributes. The first of these is to provide a distance through which the force communicating element must travel before the function of making or breaking the switch contact is accomplished. The second feature is to provide the outermost layer and the force-conveying components at the individual switch sites with structural characteristics sufficient to give the user tactile feedback when a switch has been activated or deactivated.
Unfortunately, the aspects of a switch that would best contribute to tactile response often must be balanced in the design process against other desirable characteristics such as features that enhance the durability of the device. Consequently, optimal tactile feel is difficult to achieve concurrently with optimal cost, manufacturability, reliability and durability. Some layered switch assemblies rely heavily if not exclusively on what has been referred to as "the oilcan effect" to provide an affirmative switch response that can be sensed by the user: i.e., a hemispherical or dome-shaped element is provided to activate or deactivate each switch, which dome snaps in when depressed by a finger to force switch contacts into communication with each other to complete an electrical circuit, and snaps out when the pressure of the finger is removed. This snapping in and out is accompanied by a popping sound or audible clicking and, hence, the response of the switch is detected by the sense of hearing as well as by the sense of touch. The oilcan effect occurs to some extent whenever a dome structure is employed as a force-conveying or circuit-completing element of a layered switch.
However, singular reliance on the oilcan effect to supply a detectable response requires that compromises be made with respect to other design features, such as those which contribute to the longevity or the duty cycle of the device and to the ability to use the device in certain environments. For example, in order to provide a popping response that is significant enough to be sensed by touch and/or hearing by a user, each dome must have a certain minimum height and the domes must be spaced away from the circuit-completing switch contacts by a distance that is great enough to allow the response of the switch-actuating elements to be detected. Traversing this distance requires a strong actuation force which results in substantial deformation of the domes when depressed. The magnitude of the pressure needed to activate such a switch may limit the class of persons who can effectively use or be satisfied with operation of the device and the degree to which each dome is deformed upon being pressed will limit the longevity of the device. In addition, the overall size of a membrane switch can circumscribe the range of equipment and machinery with which the device can be used to interface, especially in applications where minimizing size is a design factor that must be considered. To the extent the sound associated with depression of a switch is significant in providing an assembly with an affirmative response, dependency on the oilcan effect limits the environments where the switch can be effectively used to those in which the popping noise is certain to be detected.
It has also been known to use metal for the exterior layer of a membrane switch because that material tends to improve durability and allows the assembly to be placed in devices that will be operated in relatively harsh environments, for example, outdoor environments. However, this metal layer usually must be cut very thin, so that tactile feel will not be compromised when a finger presses a key, and applies a force to the switch components disposed beneath it. The thinness of the metal also can also further limit the useful life of the assembly.
Other forms of membrane switches include components that are fairly elaborate in structure, such as coverlays with multiple grooves cut out in them, which can increase cost and complexity of manufacture.
Accordingly, those concerned with the development, manufacture, and use of membrane switches have long recognized the need for a membrane switch that optimizes tactile response, but not at the expense of other important aspects of the device such as manufacturability, reliability and versatility. The present invention satisfies this need.